Many of the automobiles manufactured today are provided with a speed transducer which includes a two-pole magnet driven by a flexible cable attached to the vehicle transmission. The magnet cooperates with a speed cup to drive an indicator needle relative to a dial to indicate the speed of the vehicle. It has been proposed to replace such mechanical speedometers with gages including a pair of coils in quadrature which respond to electrical signals to establish a resultance magnetic field which positions a pointer.
U.S. Pat. No. 4,051,434 issued Sept. 27, 1977 to Douglas W. Sweet, discloses circuitry particularly suitable for processing a relatively low information rate signal such as is often generated by a speed transducer. The patented circuitry includes a first serial shift register, the content of which is increased by a fixed amount at the beginning of each cycle of the input signal to be measured. Thereafter the content of the first register is caused to exponentially decay until the next input cycle is detected where the process is repeated. At the beginning of each cycle of the input signal, the content of the first register is transferred to a second or memory register where the data is used to drive a gage or other output device.
When the circuitry of U.S. Pat. No. 4,051,434 is used in an automobile gage application an undesirable result may occur under low speed rapid deceleration conditions or upon a total loss of information from the transducer. For example, at low speeds the memory register is updated rather infrequently due to the low frequency of the input signals. This may cause the pointer of the gage to move toward zero in undesirably large increments. In the event of loss of sensor information the memory register will not be updated and accordingly the gage will register the speed corresponding to that prevailing at the time of sensor loss.